A known vertical power diode 101 as exemplary shown in FIG. 1 in top view and in FIG. 2 in cross-section includes, for example, a first layer 102 with a low (n-) doped base layer 106 and a high (n+) doped cathode layer 104 on one side 112 of the base layer 106 and a high (p+) doped anode layer 110 on the other side 108 of the base layer 106. The anode and cathode layers 110, 104 can be formed by implantation and a subsequent diffusion of dopants into an n− doped substrate (wafer).
On their outer side the cathode layer 104 and the anode layer 110 are covered with metal layers 114, 116 for electrically contacting the diode 101. The cathode layer 104 and metal layer 114 can extend to the physical edge 118 of the device 101. The anode layer 110, on the other hand, can be terminated at some distance 120 from the edge 118 in order to be able to support an electric field when reverse biased. This is done, for example, by limiting the p+ doped anode layer 110 to a central part 122 of the diode 101 and surrounding it by a field-limiting junction termination 124, sometimes also referred to as “guard ring”.
The field-limiting junction termination 124 includes a plurality of ring-shaped (p+) doped sub-regions 126 having a width w1 of, for example, between 5 and 50 μm, and being alternately implemented into a ring-shaped (n−) region 128 being part of the base layer 106 such that each of the (p+) sub-regions 126 is electrically disconnected from neighbouring (p+) sub-regions 126 as well from the (p+) doped anode layer 110. A design, such as an arrangement and dimensions of the sub-regions 126, 128, may have to be specifically optimized for each type of power diode and may depend for example on voltage, power and switching specifications for a specific power device.
The anode metal layer 116 has about the same size as the (p+) doped anode layer 110 itself. The area between the (p+) doped anode layer 110 and the (n+) doped cathode layer 104 can be defined as the active area 130 of the diode 101. It is enclosed by a circumferential area 132.
A known application for such a power diode 101 is as a free-wheeling diode in an IGBT inverter circuit. In such an application, an important part of the diode operation may appear when the diode 101 is switched-off from a conducting on-state to a blocking off-state as the IGBT is switched on.
Due to known effects which are for example described in the Applicant's patent application EP 1 909 332 A1, high voltage diodes may involve local lifetime control in the active area 130 for optimized electrical properties in blocking, switching and conduction state. Therefore, a lifetime control region 134 may be generated close to the anode side surface of the diode 101. This lifetime control region 134 can include defects forming recombination centres which may locally reduce a minority carrier lifetime within adjacent semiconductor material. For example, such defects may be generated by irradiating the anode side surface with ions like hydrogen ions or helium ions. These local irradiation defects can reduce the peak voltage generated in the diode during diode turn off, also known as reverse recovery peak, and may improve the safe operating area.
However, it has been observed that high power diodes having for example a design similar to that shown in FIGS. 1 and 2 may suffer from non-optimum electrical characteristics such as for example non-optimum electrical blocking capability.